1. Field of the Invention
The present invention relates generally to an apparatus and method for receiving data in a mobile communication system using an Adaptive Antenna Array (AAA) technique, and in particular, to an apparatus and method for receiving data using a 2-step weight generation technique.
2. Description of the Related Art
A next generation mobile communication system has evolved into a packet service communication system that transmits burst packet data to a plurality of mobile stations (MSs). The packet service communication system has been designed to be suitable for the transmission of mass data. Such a packet service communication system has been developing for high-speed packet service. In this regard, the 3rd Generation Partnership Project (3GPP), a standardization organization for asynchronous communication technique, proposes a High Speed Downlink Packet Access (HSDPA) to provide the high-speed packet service, while the 3rd Generation Partnership Project 2 (3GPP2), a standardization organization for synchronous communication technique, proposes a 1× Evolution Data Only/Voice (1× EV-DO/V) to provide the high-speed packet service. Both HSDPA and 1× EV-DO/V propose to provide high-speed packet service for smooth transmission of Web/Internet service, and in order to provide the high-speed packet service, a peak throughput as an well as average throughput should be optimized for smooth transmission of the packet data as well as the circuit data such as voice service data.
In order to support the high-speed transmission of packet data, a communication system employing the HSDPA (hereinafter referred to as an “HSDPA communication system”) has newly introduced 3 kinds of data transmission techniques: an Adaptive Modulation and Coding (AMC) technique, a Hybrid Automatic Retransmission Request (HARQ) technique, and a Fast Cell Selection (FCS) technique. The HSDPA communication system increases a data rate using the AMC, HARQ and FCS techniques. As another communication system for increasing a data rate, there is a communication system using the 1× EV-DO/V (hereinafter referred to as a “1× EV-DO/V communication system”). The 1× EV-DO/V communication system also increases a data rate to secure system performance. Aside from the new techniques such as AMC, HARQ and FCS, there is a Multiple Antenna technique as another technique for coping with the limitation in assigned bandwidth, i.e. increasing a data rate. The Multiple Antenna technique can overcome the limitation of bandwidth resource in a frequency domain because it utilizes a space domain.
The Multiple Antenna technique will be described herein below. A communication system is constructed such that a plurality of MSs communicate with each other via one base station (BS). When the BS performs a high-speed data transmission to the MSs, a fading phenomenon occurs due to a characteristic of radio channels. In order to overcome the fading phenomenon, a Transmit Antenna Diversity technique, a kind of the Multiple Antenna technique, has been proposed. The Transmit Antenna Diversity refers to a technique for transmitting signals using at least two transmission antennas, i.e. multiple antennas, to minimize a loss of transmission data due to a fading phenomenon, thereby increasing a data rate. The Transmit Antenna Diversity will be described herein below.
Generally, in a wireless channel environment in a mobile communication system, unlike in a wired channel environment, a transmission signal is actually distorted due to several factors such as multipath interference, shadowing, wave attenuation, time-varying noise, interference, etc. Fading caused by the multipath interference is closely related to the mobility of a reflector or a user (or aMS), and actually, a mixture of a transmission signal and an interference signal is received. Therefore, the received signal suffers from severe distortion during its actual transmission, reducing performance of the entire mobile communication system. The fading may result in the distortion in the amplitude and the phase of the received signal, preventing high-speed data communication in the wireless channel environment. Many studies are being conducted in order to resolve the fading. In conclusion, in order to transmit data at a high speed, the mobile communication system must minimize a loss due to a characteristic of a mobile communication channel such as fading, and interference of an individual user. As a technique for preventing unstable communication due to the fading, a diversity technique is used, and multiple antennas are used to implement a Space Diversity technique, one type of the diversity technique.
The Transmit Antenna Diversity is popularly used as a technique for efficiently resolving the fading phenomenon. The Transmit Antenna Diversity receives a plurality of transmission signals that have experienced an independent fading phenomena in a wireless channel environment, thereby coping with distortion caused by the fading. The Transmit Antenna Diversity is classified into Time Diversity, Frequency Diversity, Multipath Diversity, and Space Diversity. In other words, a mobile communication system must cope well with the fading phenomenon that severely affects communication performance, in order to perform the high-speed data communication. The fading phenomenon must be overcome because it reduces the amplitude of a received signal up to several dB to tens of dB. In order to overcome the fading phenomenon, the above diversity techniques are used. For example, Code Division Multiple Access (CDMA) technique adopts a Rake receiver that can achieve diversity performance using a delay spread of the channel. The Rake receiver is a kind of a Receive Diversity technique for receiving multipath signals. However, the Receive Diversity used in the Rake receiver is disadvantageous in that it cannot achieve a desired diversity gain when the delay spread of the channel is relatively small.
The Time Diversity technique efficiently copes with burst errors occurring in a wireless channel environment using interleaving and coding, and is generally used in a Doppler spread channel. Disadvantageously, however, the Time Diversity can hardly obtain the diversity effects in a low-speed Doppler spread channel. The Space Diversity technique is generally used in a channel with a low delay spread such as an indoor channel and a pedestrian channel which is a low-speed Doppler spread channel. The Space Diversity is a technique for achieving a diversity gain using at least two antennas. In this technique, when a signal transmitted via one antenna is attenuated due to fading, a signal transmitted via another antenna is received, thereby acquiring a diversity gain. The Space Diversity is classified into Receive Antenna Diversity using a plurality of reception antennas and Transmit Antenna Diversity using a plurality of transmission antennas.
A Receive-Adaptive Antenna Array (Rx-AAA) technique, a kind of the Receive Antenna Diversity technique, will be described herein below.
In the Rx-AAA technique, by calculating a scalar product of an appropriate weight vector and a signal vector of a reception signal received via an antenna array comprised of a plurality of reception antennas, a signal received in a direction desired by a receiver is maximized in its level and a signal received in a direction not desired by the receiver is minimized in its level. As a result, the Rx-AAA technique amplifies only a desired reception signal to a maximum level thereby maintaining a high-quality call and causing an increase in the entire system capacity and service coverage.
Although the Rx-AAA technique can be applied to both a Frequency Division Multiple Access (FDMA) mobile communication system and a Time Division Multiple Access (TDMA) mobile communication system, it will be assumed herein that the Rx-AAA technique is applied to a communication system using CDMA techniques (hereinafter referred to as a “CDMA communication system”).
FIG. 1 is a block diagram illustrating a structure of a BS receiver in a conventional CDMA mobile communication system. Referring to FIG. 1, the BS receiver is comprised of N reception antennas (Rx_ANT) of a first reception antenna 111, a second reception antenna 121, . . . , and an Nth reception antenna 131, N radio frequency (RF) processors of a first RF processor 112, a second RF processor 122, . . . , and an Nth RF processor 132, being mapped to the corresponding reception antennas, N multipath searchers of a first multipath searcher 113, a second multipath searcher 123, . . . , and an Nth multipath searcher 133, being mapped to the corresponding RF processors, L fingers of a first finger 140-1, a second finger 140-2, . . . , and an Lth finger 140-L, for processing L multipath signals searched by the multipath searchers, a multipath combiner 150 for combining multipath signals output from the L fingers, a deinterleaver 160, and a decoder 170.
Signals transmitted by transmitters in a plurality of MSs are received at the N reception antennas over a multipath fading radio channel. The first reception antenna 111 outputs the received signal to the first RF processor 112. Each of the RF processors is comprised of an amplifier, a frequency converter, a filter, and an analog-to-digital (A/D) converter, and processes an RF signal. The first RF processor 112 RF-processes a signal output from the first reception antenna 111 to convert the signal into a baseband digital signal, and outputs the baseband digital signal to the first multipath searcher 113. The first multipath searcher 113 separates L multipath components from a signal output from the first RF processor 112, and the separated L multipath components are output to the first finger 140-1 to the Lth finger 140-L, respectively.
The first finger 140-1 to the Lth finger 140-L, being mapped to the L multiple paths on a one-to-one basis, process the L multipath components. Because the L multiple paths are considered for each of the signals received via the N reception antennas, signal processing must be performed on N×L signals, and among the N×L signals, signals on the same path are output to the same finger.
Similarly, the second reception antenna 121 outputs the received signal to the second RF processor 122. The second RF processor 122 RF-processes a signal output from the second reception antenna 121 to convert the signal into a baseband digital signal, and outputs the baseband digital signal to the second multipath searcher 123. The second multipath searcher 123 separates L multipath components from a signal output from the second RF processor 122, and the separated L multipath components are output to the first finger 140-1 to the Lth finger 140-L, respectively.
In this same manner, the Nth reception antenna 131 outputs the received signal to the Nth RF processor 132. The Nth RF processor 132 RF-processes a signal output from the Nth reception antenna 131 to convert the signal into a baseband digital signal, and outputs the baseband digital signal to the Nth multipath searcher 133. The N th multipath searcher 133 separates L multipath components from a signal output from the Nth RF processor 132, and the separated L multipath components are output to the first finger 140-1 to the Lth finger 140-L, respectively.
In this way, among the L multipath signals for the signals received via the N reception antennas, the same multipath signals are input to the same fingers. For example, first multipath signals from the first reception antenna 111 to the Nth reception antenna 131 are input to the first finger 140-1. In the same manner, Lth multipath signals from the first reception antenna 111 to the Nth reception antenna 131 are input to the Lth finger 140-L. The first finger 140-1 to the Lth finger 140-L are different only in signals input thereto and output therefrom, and are identical in structure and operation. Therefore, only the first finger 140-1 will be described for simplicity.
The first finger 140-1 is comprised of N despreaders of a first despreader 141, a second despreader 142, . . . , and an Nth despreader 143, being mapped to the N multipath searchers, a signal processor 144 for calculating a weight vector for generating a reception beam using signals received from the N despreaders, and a reception beam generator 145 for generating a reception beam using the weight vector calculated by the signal processor 144.
A first multipath signal output from the first multipath searcher 113 is input to the first despreader 141. The first despreader 141 despreads the first multipath signal output from the first multipath searcher 113 with a predetermined spreading code, and outputs the despread multipath signal to the signal processor 144 and the reception beam generator 145. Here, the despreading process is called “temporal processing.” Similarly, a first multipath signal output from the second multipath searcher 123 is input to the second despreader 142. The second despreader 142 despreads the first multipath signal output from the second multipath searcher 123 with a predetermined spreading code, and outputs the despread multipath signal to the signal processor 144 and the reception beam generator 145. In the same way, a first multipath signal output from the Nth multipath searcher 133 is input to the Nth despreader 143. The Nth despreader 143 despreads the first multipath signal output from the Nth multipath searcher 133 with a predetermined spreading code, and outputs the despread multipath signal to the signal processor 144 and the reception beam generator 145.
The signal processor 144 receives the signals output from the first despreader 141 to the Nth despreader 143, and calculates a weight set Wk for generation of reception beam. Here, a set of first multipath signals output from the first multipath searcher 113 to the Nth multipath searcher 133 will be defined as “Xk.” The first multipath signal set Xk represents a set of first multipath signals received via the first reception antenna 111 to the Nth reception antenna 131 at a kth point, and the first multipath signals constituting the first multipath signal set Xk are all vector signals. The weight set Wk represents a set of weights to be applied to the first multipath signals received via the first reception antenna 111 to the Nth reception antenna 131 at the kth point, and the weights constituting the weight set Wk are all vector signals.
A set of signals determined by despreading all of the first multipath signals in the first multipath signal set Xk will be defined as yk. The despread signal set yk of the first multipath signals represents a set of signals determined by despreading the first multipath signals received via the first reception antenna 111 to the Nth reception antenna 131 at the kth point, and the despread signals constituting despread signal set yk of the first multipath signals are all vector signals. Here, for the convenience of explanation, the term “set” will be omitted, and the underlined parameters represent sets of corresponding elements.
Each of the first despreaders 141 to the Nth despreaders 143 despreads the first multipath signal Xk with a predetermined despreading code, so that the reception power of a desired reception signal is greater than the reception power of an interference signal by a process gain. Here, the despreading code is identical to the spreading code used in the transmitters of the MSs.
As described above, the despread signal yk of the first multipath signal Xk is input to the signal processor 144. The signal processor 144 calculates a weight Wk with the despread signal yk of the first multipath signal Xk, and outputs the weight Wk to the reception beam generator 145. As a result, the signal processor 144 calculates the weight Wk including a total of N weight vectors applied to the first multipath signal Xk output from the first reception antenna 111 to the Nth reception antenna 131, with the despread signals yk of a total of N first multipath signals output from the first reception antenna 111 to the Nth reception antenna 131. The reception beam generator 145 receives the despread signals yk of a total of the N first multipath signals Xk and a total of the N weight vectors Wk. The reception beam generator 145 generates a reception beam with a total of the N weight vectors Wk, calculates a scalar product of the despread signal yk of the first multipath signal Xk and the weight Wk corresponding to the reception beam, and outputs the result as an output zk of the first finger 140-1. The output zk of the first finger 140-1 can be expressed aszk=wkHyk  (1)In Equation (1), H denotes a Hermitian operator, i.e. a conjugate-transpose. A set zk of output signals zk from L fingers in the BS receiver is finally input to the multipath combiner 150.
Although only the first finger 140-1 has been described, the other fingers are also equal to the first finger 140-1 in operation. Therefore, the multipath combiner 150 combines the signals output from the first finger 140-1 to the Lth finger 140-L, and outputs the combined signal to the deinterleaver 160. The deinterleaver 160 deinterleaves the signal output from the multipath combiner 150 in a deinterleaving method corresponding to the interleaving method used in the transmitter, and outputs the deinterleaved signal to the decoder 170. The decoder 170 decodes the signal output from the deinterleaver 160 in a decoding method corresponding to the encoding method used in the transmitter, and outputs the decoded signal as final reception data.
The signal processor 144 calculates a weight Wk such that a Mean Square Error (MSE) of a signal received from a MS transmitter, desired to be received by a predetermined algorithm, becomes minimized. The reception beam generator 145 generates a reception beam using the weight Wk generated by the signal processor 144, and the process of generating a reception beam so that MSE becomes minimized is called “spatial processing.” Therefore, when the Rx-AAA technique is used in a CDMA mobile communication system, temporal processing and spatial processing are simultaneously performed. The operation of simultaneously performing temporal processing and spatial processing is called “spatial-temporal processing.”
The signal processor 144 receives multipath signals despread for each finger in the above-stated manner, and calculates a weight capable of maximizing a gain of the Rx-AAA technique according to a predetermined algorithm. The signal processor 144 minimizes the MSE. Therefore, a recent study is actively conducted on a weight calculation algorithm for adaptively minimizing the MSE. However, the weight calculation algorithm for adaptively minimizing the MSE is an algorithm for reducing errors on the basis of a reference signal, and this algorithm supports a Consultant Modulus (CM) technique and a Decision-Directed (DD) technique as a blind technique, when there is no reference signal.
However, the algorithm for reducing errors on the basis of a reference signal is hard to converge into a minimum MSE value desired by the system in an environment where a channel such as a fast fading channel suffers from a rapid change, or an environment where a high-order modulation scheme such as 16-ary quadrature amplitude modulation (16 QAM) is used. Even though it converges into a particular MSE value, the minimum MSE value is set to a relatively large value. When the minimum MSE value is set to a relatively large value, a gain occurring by the use of the Rx-AAA technique is reduced. Therefore, this algorithm is not suitable for a high-speed data communication system.